1. Field of Technology
The present invention generally relates to processes for producing fuel. More specifically, the invention relates to the production of fuel using alcohol derived from any source as a feedstock or co-feedstock with coal, biomass, petcoke, or natural gas in a synthesis gas refinery.
2. Background of the Invention
It is well known that internal combustion engines have revolutionized transportation following the invention thereof during the last decades of the 19th century. While others, including Karl Benz and Gottleib Wilhelm Daimler, invented and developed engines using electric ignition of fuel such as gasoline, Rudolf C. K. Diesel invented and built the engine named for him which employs compression for auto-ignition of the fuel, enabling utilization of low-cost organic fuels. Modern high performance diesel engines require increasingly advanced fuel compositions.
Currently, the majority of fuels for transportation are derived from natural petroleum. Petroleum remains the world's main source of hydrocarbons used as fuel and petrochemical feedstock. While compositions of natural petroleum or crude oils are significantly varied, all crude contains sulfur compounds and most crude contains nitrogen compounds which may also contain oxygen, but the oxygen content of most crude is low.
Fuels such as diesel which are used widely in automotive transport and for providing power for heavy duty equipment are of great interest due to their high fuel economy. However, one of the problems with diesel fuels is that, when such fuels are burned in internal combustion engines, pollutants in the exhaust gases are emitted into the environment. Some of the most common pollutants in diesel exhausts are nitric oxide and nitrogen dioxide (hereafter abbreviated as “NOx”), hydrocarbons and sulfur dioxide, and to a lesser extent carbon monoxide. In addition, diesel powered engines also generate a significant amount of particulate emissions (PM) which include soot, adsorbed hydrocarbons, and sulfates, which are usually formed due to the incomplete combustion of the fuel and cause of dense black smoke emitted by such engines through the exhaust.
Crude oil is seldom used as produced at the well, but is converted in oil refineries into a wide range of fuels and petrochemical feedstocks. Fuels for transportation are typically produced by processing and blending distilled fractions from the crude to meet particular end use specifications. Because most of the crude available today in large quantity is high in sulfur, the distilled fractions must be desulfurized to yield products which meet performance specifications and/or environmental standards. As mentioned above, sulfur-containing organic compounds in fuels continue to be a major source of environmental pollution. During combustion they are converted to sulfur oxides which, in turn, give rise to sulfur oxyacids and also contribute to particulate emissions.
Distilled fractions used for fuel or a blending component of fuel for use in compression ignition internal combustion engines (diesel engines) are middle distillates that usually contain from about 1 to 3 percent by weight sulfur. In the past a typical specifications for diesel fuel was a maximum of 0.5 percent by weight. By 1993 legislation in Europe and United States limited sulfur in diesel fuel to 0.3 weight percent. By 1996 in Europe and United States, and 1997 in Japan, maximum sulfur in diesel fuel was reduced to no more than 0.05 weight percent. This world-wide trend must be expected to continue to even lower levels for sulfur.
Introduction of new emission regulations has prompted significant interest in catalytic exhaust treatment. Challenges of applying catalytic emission control for the diesel engine, particularly the heavy-duty diesel engine, are significantly different from the spark ignition internal combustion engine (gasoline engine) due to two factors. First, the conventional three way catalyst (TWC) is ineffective in removing NOx emissions from diesel engines, and second, the need for particulate control is significantly higher than with the gasoline engine.
Several exhaust treatment technologies are emerging for control of diesel engine emissions, and in all sectors the level of sulfur in the fuel affects efficiency of the technology. Sulfur is a catalyst poison that reduces the activity of catalysts. High fuel sulfur also creates a secondary problem of particulate emission during catalytic control of diesel emissions, due to catalytic oxidation of sulfur and reaction with water to form a sulfate mist. This mist is collected as a portion of particulate emissions. While an increase in combustion temperature can reduce particulate, this leads to an increase in NOx emission by the well-known Zeldovitch mechanism. Thus, it becomes necessary to trade off particulate and NOx emissions to meet emissions legislation.
Available evidence strongly suggests that ultra-low sulfur diesel (ULSD) is a significant technology enabler for catalytic treatment of diesel exhaust to control emissions. Fuel sulfur levels of below 15 ppm are likely required to achieve particulate levels below 0.01 g/bhp-hr. Such levels would be compatible with catalyst combinations for exhaust treatment now emerging, which have shown capability to achieve NOx emissions around 0.5 g/bhp-hr. Furthermore, NOx trap systems are extremely sensitive to fuel sulfur and available evidence suggests that they need sulfur levels below 10 ppm to remain active.
Conventional hydrodesulfurization (HDS) catalysts can be used to remove a major portion of the sulfur from petroleum distillates for the blending of refinery transportation fuels, but they are not efficient for removing sulfur from compounds where the sulfur atom is sterically hindered, as in multi-ring aromatic sulfur compounds. This is especially true where the sulfur heteroatom is doubly hindered (e.g., 4,6-dimethyldibenzothiophene). Using conventional hydrodesulfurization catalysts at high temperatures would cause yield loss, faster catalyst coking, and product quality deterioration (e.g., color). High pressure utilization also incurs a large capital outlay.
In order to meet strict specifications, hindered sulfur compounds will also have to be removed from distillate feedstocks and products. There is a pressing need for economical removal of sulfur from distillates and other hydrocarbon products and the art provides many processes said to remove sulfur from distillate feedstocks and products. These methods have been of limited utility, however, since only a rather low degree of desulfurization is achieved. Also, substantial loss of valuable products may result due to cracking and/or coke formation during the utilization of these methods.
As described in U.S. Pat. No. 6,824,574 to O'Rear, et al., the increased demand for middle distillate transportation fuels such as jet fuel and diesel fuel has provided the incentive for expanding the production of these fuels by converting natural gas, coal and heavy petroleum fractions. The growing importance of alternative energy sources has thus brought a renewed interest in Fischer-Tropsch synthesis as one of the more attractive direct and environmentally acceptable paths to high quality transportation fuels, as Fischer-Tropsch synthesis may be used to create Fischer-Tropsch diesel comprising very low levels or substantially no sulfur.
The Fischer-Tropsch synthesis for processing these resources into distillate fuels involves a Fischer-Tropsch reaction whereby synthesis gas containing essentially hydrogen and carbon monoxide is converted into highly linear hydrocarbonaceous products containing paraffins, olefins and oxygenates such as acids and alcohols. The linear paraffins are converted into isoparaffinic distillate fuel components using known Fischer-Tropsch product upgrading procedures such as hydrotreating, hydrocracking and hydroisomerization dewaxing. The isoparaffinic distillate fuels have excellent burning properties, including high jet smoke points and high diesel cetane numbers.
Even in newer, high performance diesel engines, combustion of conventional fuel produces smoke in the exhaust. Oxygenated compounds and compounds containing few or no carbon-to-carbon chemical bonds, such as methanol and dimethyl ether, are known to reduce smoke and engine exhaust emissions. However, most such compounds have high vapor pressure and/or are nearly insoluble in diesel fuel, and they have poor ignition quality, as indicated by their cetane numbers. Furthermore, other methods of improving diesel fuels by chemical hydrogenation to reduce their sulfur and aromatics contents, also cause a reduction in fuel lubricity. Diesel fuels of low lubricity may cause excessive wear of fuel injectors and other moving parts which come in contact with the fuel under high pressures.
Much current work has concentrated on the production of ethanol, which has been viewed as a final fuel or a fuel blending component suitable for enhancing environmentally-friendliness of fuels and favorably altering the fuels, e.g. to reduce the smoke point and/or emissions. For example, as described in U.S. Pat. No. 7,208,022 to Corkwell, et al., using ethanol in gasoline is well established around the world. Mixing alcohol, e.g. ethanol, with diesel has proven problematic due to the phase separation which occurs, leading to corrosion, etc., and the need for emulsions. Indeed, ethanol-diesel fuel mixtures have suffered in a variety of performance areas: stability (especially in the presence of water), corrosion, reduced power, lubricity, and low temperature properties. Ethanol-diesel fuel mixtures, especially in the presence of water and/or low temperatures, tend to be unstable resulting in separation to polar and nonpolar phases. The corrosive properties of ethanol have been assigned to the instability of the mixture when exposed to contaminant water in the fuel delivery system. Thus, while the use of ethanol in diesel fuel systems can offer economic and environmental advantages from a renewable fuel point of view, the presence of water presents complex technical problems relating to storage and use of such fuels.
As methods of producing ethanol and other mixed alcohols from various readily-available sources prove increasingly economical, methods of producing suitable fuels therefrom are needed. Accordingly, there is a need in the art for a method of producing environmentally-friendly fuels from alcohols. The fuel desirably has a low sulfur content leading to a relative ease of removing particulate matter (PM) upon combustion, e.g. via catalytic conversion. Such a fuel may desirably be used alone or blended with other fuels or fuel components.